23 Replies Latest reply on May 3, 2019 8:01 AM by abrain

    Clearance - 30v 3A TENMA PSU

    14rhb

      I regularly look through the clearance lines on CPC Farnell (a UK subsidiary of Farnell/Avnet) and currently they have a Tenma 30v/3A PSU for £51.84 (incl. tax and shipping)....not bad if someone is looking for a reasonable entry level PSU.

       

      See https://cpc.farnell.com/tenma/72-2705/power-supply-0-30v-programmable/dp/IN0763769

       

      Rod

        • Re: Clearance - 30v 3A TENMA PSU
          stevesmythe

          Well-spotted Rod. I have been thinking about getting a decent bench PSU for a while. Might take the plunge.

            • Re: Clearance - 30v 3A TENMA PSU
              14rhb

              It came up under the clearance section with a discount of something like 45% (to arrive at that price) however the direct link doesn't seem to highlight the fact it is a reduced price at £51. However I still think it looks like great value for a hobbyist setup. If you do purchase one I'd be interested to know what you make of it; I recently bought a 15v/2A Duratool PSU from CPC Farnell (Duratool D03232 Power Supply ) so could not really justify getting another so soon !

               

              Rod

              2 of 2 people found this helpful
            • Re: Clearance - 30v 3A TENMA PSU
              shabaz

              Thanks for the info!

              That looks useful, and I could do with a small linear supply.. it's got fairly accurate readback capability it seems, and also the supply is programmable..

              I could not resist at that price, so I have ordered one.

                • Re: Clearance - 30v 3A TENMA PSU
                  stevesmythe

                  I have succumbed too. After all, it was my birthday last week (honestly). When I first looked, there were only two in stock, so I thought I'd better get a shift on!

                    • Re: Clearance - 30v 3A TENMA PSU
                      14rhb

                      Happy Birthday Steve

                       

                      Rod

                      • Re: Clearance - 30v 3A TENMA PSU
                        shabaz

                        Hi Steve,

                         

                        Happy Birthday!

                         

                        Also, the supply arrived this morning, so hopefully you'll have got yours today too. I won't get a chance to try it out properly until tonight or later in the week, but I did switch it on and try it for five minutes..

                        it's all-steel construction with a plastic front panel, very heavy since it's got the transformer, and you can hear the relays clicking through the ranges. The user interface, since it has just 2 rotary knobs with buttons, is ok-ish, I've seen a lot worse! but it will be interesting to see what you think of it too. At first glance there's a few typos and mistakes in the manual, but nothing we cannot figure out I'm sure!

                        2 of 2 people found this helpful
                          • Re: Clearance - 30v 3A TENMA PSU
                            stevesmythe

                            Thanks!

                             

                            I hesitated before ordering and missed the cut-off, so mine won't arrive today. I'm glad there is a manual (even with typos and mistakes). I wasn't sure if there would be. My power supply needs are fairly simple, so I'm sure it will do me nicely, but I'm looking forward to not having to hunt around for a suitable power supply when I get a new gizmo!

                            • Re: Clearance - 30v 3A TENMA PSU
                              stevesmythe

                              Mine came today .

                               

                              First impressions are favourable, although I would have appreciated a longer mains lead. As I don't actually have a bench, my power sockets are not just above the bench (I have a desk with sockets in the skirting board), so I can't blame them. It's only a "kettle lead" so easily rectified (no pun intended).

                               

                              Just a slight hum from the transformer.

                               

                              It's a bit retro to be provided with an actual CD nowadays. No instructions or manual for the software (imaginatively entitled "72-2705") but seems straightforward.The USB driver installed automatically on one of my Windows 10 PCs, but another of my PCs doesn't like the USB device at all. However, that was also easily rectified as I used one of those cheap CH340 Serial-USB converters. I have an actual serial port on that PC, so I need to dig out a proper serial to serial cable for it (assuming I didn't chuck them all away).

                               

                              [edit] After installing the software and playing with it via the serial-USB adapter, my PC mysteriously decided to recognise the Tenma's USB port, so all is good now!

                              4 of 4 people found this helpful
                                • Re: Clearance - 30v 3A TENMA PSU
                                  shabaz

                                  Hi Steve,

                                   

                                  Oh excellent, that's great to hear it functions on Windows 10.. I will try to upload the CD to the cloud somewhere, since I'm bound to lose it!

                                  • Re: Clearance - 30v 3A TENMA PSU
                                    shabaz

                                    Hi Steve,

                                     

                                    Hehe I had the same issue, the supplied kettle-lead is too short : )

                                    I've backed-up the user manual and CD contents, in case we ever lose them.

                                    My PC (running Windows 10) didn't seem to need the driver, is used the default usbser.sys automatically.

                                    Also, for Linux, plugging the USB connector into a Pi caused a /dev/ttyACM0 to appear, so that's working fine too.

                                    I tried a few commands and it gave the right responses! It's reporting back V2.5 firmware, although the user manual mentions V2.0.

                                     

                                    The only slight issue is that (like other) SCPI instruments, it's a wild west and linefeeds are sometimes not implemented as they should be, with some instruments wanting carriage-returns too, and so on.

                                    In the case of this device, it seems to not want the control character, and it ignores them which I can live with, but the responses contain no linefeed either : ( That means either a delay needs to be implemented, or characters counted/parsed as they arrive. Anyway, since this is a simple device, it can be easily worked around - the numbers are formatted, so won't be variable-length.

                                    In summary, the programmable capability is usable, so we can write simple code to do stuff if desired!

                                     

                                    This code is working for me on the pi.. (first type pip install pyserial ):

                                     

                                    #!/usr/bin/python
                                    ####################################
                                    # PSU test
                                    ####################################
                                    # definitions
                                    import sys
                                    import serial
                                    
                                    # main program
                                    if (len(sys.argv)>1):
                                      psu=serial.Serial(sys.argv[1], 9600, timeout=1)
                                    else:
                                      psu=serial.Serial('/dev/ttyACM0', 9600, timeout=1)
                                    
                                    print "writing.."
                                    psu.write(b'*IDN?')
                                    print "reading.."
                                    resp=psu.read(64);
                                    print "result is "+resp+" length is "+str(len(resp))
                                    

                                     

                                     

                                    I've not connected up the PSU output to any equipment yet, but just playing around with the UI, I'm happy with that aspect! There's two UI modes, I set mode 2 (press both rotary buttons for about 1 second to toggle between them, although the manual says 3 seconds) and to me that's more usable. If I ever get around to opening the supply, it would be nice to perhaps change out the lower red LEDs to a different color, so that red is only for CC/CV LEDs, to more easily see at a glance if the output is on, by looking for the presence of red.

                                     

                                    It's also cool that 'electronic fuse' capability can be enabled if desired, aka 'over current protection' mode, i.e. rather than transition to CC mode, just cut out the output if the set current is exceeded. that's really nice to have, my old PSU is missing that.

                                     

                                    Thanks 14rhb for spotting this supply, it's looking promising... If it's good once tested I hope to buy another to get dual-rail capability.

                                    3 of 3 people found this helpful
                                    • Re: Clearance - 30v 3A TENMA PSU
                                      stevesmythe

                                      Yay! Found a couple of nice long kettle leads AND a long RS232 serial cable (male DB9 to female DB9) in the garage. Frees up a USB port and no annoying USB driver issues to contend with. I have the same V2.5 firmware as you (not surprisingly).

                              • Re: Clearance - 30v 3A TENMA PSU
                                shabaz

                                I did some initial measurements, and it's very good : )

                                I did the measurements at 5V (since that's a popular voltage!) and at 30V.

                                5V set voltage:

                                At no load, the output was 5.0088V, and at 3A load it was 4.9969V

                                30V set voltage:

                                At no load, output was 30.039V, and at 3A load it was 30.029V

                                 

                                So, set voltage is spot-on, and it seems to handle the max current fine too. Noise was low, I couldn't observe any ripple.

                                This is noise at 30V with no load attached, and 3A load  (there is some faint garbage just visible on the trace background, but some of that will be due to scope probing, and some of it can be removed with a ferrite):

                                 

                                Switching the output on (i.e. power supply is already on, but output is off, and it is switched on via SCPI), with no load attached:

                                Switching it off:

                                 

                                Tests with a constant current load attached:

                                Next I tried the same thing as above but with a 3A constant current load attached:

                                And switching the output off:

                                 

                                Tests with a resistive load attached (home-made load, Building a Miniature 300W Speaker Simulator  ):

                                I set the supply to 24V and attached a 8 ohm load, so that the expected current was 3A. I switched the output on via SCPI:

                                As can be seem, there was an anomaly just after 15V or so. Maybe it is switching ranges there, since maybe the time period seems consistent with typical relays switching. Still, this looks great, no overshoot, and the output is flat within 20msec or so.

                                Here's the output switching off:

                                That anomaly at switch-on happens at different voltages too, down to about 15V. Here it is at 16V:

                                I then set the load to 4 ohms, and set the supply to 5V and switched the output on via SCPI. Again, no overshoot, and output was flat within milliseconds:

                                I tried lower popular voltages too, like 3.3V:

                                and 1.8V:

                                 

                                So, all this looks great. There were some negatives though:

                                 

                                Power off/on glitches

                                If the supply is powered on or off, sometimes there is some output visible.

                                Here it is powering off. This was the worst-case I could capture. Most of the time, it was less than this, but (say) 5% of the time switching off, I saw this:

                                Powering on produced junk for a shorter time:

                                Adding the largest ferrites I had (not very large - one was about 1 inch OD and 0.5inch long, and the other was about 1.5 inch long and 3/4 inch OD) to the output wires, improved things a lot.

                                With a load attached, it's better:

                                 

                                Other issues:

                                The main issue is that there's no easy way to switch the output on or off, without powering up or powering down the entire supply. However, if the output is switched off via SCPI, then it seems to retain that forever*, and the output can be switched on by holding the voltage knob down to lock it, and then holding it down to unlock it. But, there's no way I can see to easily switch the output off from the front panel : ( It requires SCPI to do that.

                                Maybe I'm missing some trick on the front panel. (EDIT - problem resolved, see Steve's comment regarding Mode 3).

                                The workaround is to always use SCPI to control the output on/off. That could be an annoyance for some, it's certainly not convenient. Worst-case, someone could write some Arduino code to control via the rear serial, to provide this functionality.

                                 

                                * I could be wrong here.. I've not spent enough time to understand the behaviour totally - these were just quick experiments to get to know the device.

                                 

                                To do:

                                I've not tried to examine behavior under overload, with CC mode nor the electronic fuse (OCP) mode, nor changing load. Also, I've not explored how accurate the current and voltage readback capability is. These latter two items are nice accoutrements, that I figured can be measured later.

                                 

                                Summary:

                                The performance seems to be great so far! It is very low noise, and practically no ripple even at 3A load. The voltage set is accurate. I'm really happy with it. The lack of front-panel button to easily switch the output on or off is a limitation, but I cannot help feeling I'm maybe missing some button trick to do that (EDIT - problem resolved, see Steve's comment regarding Mode 3).. The problem is not unsolvable, there are workarounds, but it would be nice for this to be easily controlled from the front panel, since the capability is accessible via SCPI using the rear-panel USB or serial connections.

                                5 of 5 people found this helpful
                                  • Re: Clearance - 30v 3A TENMA PSU
                                    stevesmythe

                                    Great work shabaz! Your tests show its performance to be better than I expected at the price.

                                     

                                    I agree about not being able to switch the output off without powering down. I had already been searching around for a way to do that. It's a bit dangerous I think (for the components I mean).

                                      • Re: Clearance - 30v 3A TENMA PSU
                                        shabaz

                                        Hi Steve,

                                         

                                        Thanks!

                                        I've got a theoretical solution, but it would need a hardware mod : (

                                        The idea is, to use any small chip (e.g. Arduino or MSP430) to act as a pass-through for serial transmit. In other words, it would have one input, and one output, and it would be inserted in the path for the data received by the serial port on the PSU. No need to implement dual UARTs, the pass-through would be done via interrupt or forever loop, since the baud rate is low. It would just repeat whatever logic level is received, with no regard to protocol.

                                        But, if a button was pressed (connected to another input on the Arduino) then temporarily the pass-through would be broken, and the Arduino would send the output on/off command and then re-connect the pass-through. Like an ability to send an out-of-band message via button-press, but retain serial comms to any attached PC at all other times.

                                        I like the PSU enough to want to buy another, it could be worth implementing this feature, if there really isn't a way to do it from the front panel today.

                                         

                                        (EDIT - problem resolved, see Steve's comment regarding Mode 3).

                                        2 of 2 people found this helpful
                                      • Re: Clearance - 30v 3A TENMA PSU
                                        stevesmythe

                                        Actually, there's Mode 3. I think that does what we want. [added edit] "At this time, the CURRENT adjustment knob becomes the output knob".

                                        2 of 2 people found this helpful
                                        • Re: Clearance - 30v 3A TENMA PSU
                                          14rhb

                                          Hi Shabaz,

                                           

                                          Those are some great measurements you have undertaken, very useful reference to anyone looking at buying a supply and wondering how to compare it. I'm sure Tenma will be pleased to see their product looking so favourably.  I'm glad you and Steve seem to have picked up such a useful and well priced bench unit. It is always worth having a look through CPC Farnell and searching on 'clearance' to see what is available. I usually skim through the top items in a few categories that are 50%+ reduced. I picked up a reel of PLA (750g 2.85mm) for £11 (UK prices) incl VAT and postage plus some SMD basic diodes for 2p each!

                                           

                                          Rod

                                          1 of 1 people found this helpful
                                            • Re: Clearance - 30v 3A TENMA PSU
                                              shabaz

                                              Hi Rod,

                                               

                                              I'm definitely going to look out more often for these offers! This is great to hear that there's other interesting stuff too.

                                              The Tenma supply is back in stock in a few weeks time, so that's good.

                                              • Re: Clearance - 30v 3A TENMA PSU
                                                stevesmythe

                                                I'm very happy with the power supply (even more so, now that Shabaz has tested it), so thanks again for letting us know.

                                                 

                                                Yes, I lurk in Bargain Corner too! (But I don't normally look in the "Test Equipment" part of "Bargain Corner" which is why I missed this power supply). It's also worth glancing at the various printed CPC catalogues that seem to arrive on my doorstep every Saturday. Sometimes they have offer codes in there which give you a much lower price than if you search for the same item on the online catalogue (e.g. packs of 40 AA batteries are often half-price in the catalogues).

                                                2 of 2 people found this helpful
                                              • Re: Clearance - 30v 3A TENMA PSU
                                                abrain

                                                Hi Shabaz, that’s a nice set of measurements! I was wondering what you’re using for the 3A load? I’ve got a project driving a very inductive load at a constant current of around 2A using PWM at around 500Hz, and want an electronic load for testing with, so any recommendations would be much appreciated!

                                                 

                                                A

                                                  • Re: Clearance - 30v 3A TENMA PSU
                                                    shabaz

                                                    Hi!

                                                    The constant current was using a BK8600 load (review here), it's got an easy-to-use front-panel interface, and simple to automate if desired too. If it's for personal use, there's a lower-cost Tenma load, but I've not tried it.

                                                    It's my third BK Precision piece of equipment, it seems very usable to me. Most of the electronic loads (including the Tenma one it seems) use large binding posts, so normal 4mm banana test leads may not fit, but the end of the test lead could be cut off and a M6 lug crimped on.

                                                    2 of 2 people found this helpful